Early detection of breast cancer is a primary goal in cancer imaging and it is important to identify tumors at an early stage of growth to identify their type. Standard x-ray mammography does not always provide adequate diagnosis; e.g., in cases such as dense or radiation-scarred breasts, and often does not identify tumor presence until significant growth has occurred. Alternative methods are being developed to circumvent these problems, including MRI, ultrasound, scintigraphy, and other methods. A novel method using biomagnetic sensors to detect superparamagnetic particles targeted to tumors is described here. This technique takes advantage of a large amount of research into the use of nanoparticles containing ferric cores and coated with various agents which target tumors either through their dense vascularization or through specific antibodies. Standard biomagnetic sensors using Superconducting Quantum Interference Devices (SQUID) are capable of detecting nanograms of such materials. Research into the use of such nanoparticles, referred to as ultra-small particles of iron oxide (USPIO) indicate that this quantity is easily deliverable to tumor sites with injections orders-of-magnitude below tolerance levels and with significant tumor-to-normal tissue density ratios. This Phase I study has the specific aims of (1) constructing a breast phantom with small inclusions of USPIO agents of various types, (2) measuring the magnetic fields of magnetized USPIO inclusions in this phantom, (3) provide a preliminary design of a biomagnetic sensor system specifically for breast cancer imaging, and (4) design optimal methods for magnetizing the superparamagnetic nanoparticles whether bound in a tumor by antibodies or in microvessels of tumor vascular structure.